Most snow plows are attached to the propelling vehicle by hinge device that allows the plow to move up and down freely. The plow also must be removable so that it can be detached when it is not needed. The prior art and common practice is to engage the plow frame to the vehicle by means of two pins that must be manually inserted though a set of mating ears on the plow frame and on the vehicle.
This provides a rugged and secure method of attachment. The primary problem with this design is that it is very difficult to align the sets of ears sufficiently accurately that the pins can be inserted. Generally the plow must be moved back and forth manually until the holes align. This is quite difficult because even the lightest plows weigh over 300 pounds. The person must move the plow while at the same time attempting to insert the pin. The plow must be precisely aligned in 4 of the six positional dimensions. It must be aligned in vertical and forward and back linear dimensions and roll and yaw rotational dimensions. The alignment must be made to better than 1/32 of an inch in most cases. The two other dimensions (side to side and pitch) need not be aligned as well.
When the plow is to be detached it can be quite difficult to remove the pins. This is because the weight of the plow is resting on the pins.
The prior art has understood this problem and proposed some solutions. Referenced patents;
(a) U.S. Pat. No. 4,962,599 to Harris. PA1 (b) U.S. Pat. No. 3,987,562 to Deen et al. PA1 (c) U.S. Pat. No. 4,205,825 to Stanford. PA1 (d) U.S. Pat. No. 4,821,435 to Pester. PA1 (e) U.S. Pat. No. 4,342,163 to Hoekstra.
Harris describes a means of attachment that will automatically latch the plow frame to the vehicle frame when vehicle is moved forward against the plow frame. The latching action is made as a result of two spring loaded pins. The spring being released when plow frame and vehicle frame come together. This is a variation on the current practice and when attached, the joint will be strong and reliable.
This mechanism has the effect of reducing the accuracy that the plow needs to be aligned when mating to the vehicle. The forward dimension is taken up by moving the vehicle forward into the plow. The remaining three dimensions will be forced into alignment to a small degree by the tapered entry into the latch.
This mechanism has several disadvantages. The first is that it is relatively complicated. Such complication would cost considerably more than the standard hitch. The mechanism could be damaged if the plow were not well aligned before mating was attempted. This device provides no help in detaching the plow, the pins must be removed against the plow's weight. The plow frame must be held up from the floor manually to enable coupling.
Deen et al. show another automatic attachment device. This device is a jaw that grips a pin fixed to the vehicle frame. The jaw is shaped such that when it is struck by the pin during the attachment operation it will rotate to the closed position. The coupling is completed by the operator moving two levers to the latched position and securing them by means of a lock pin.
Like the Harris design, this mechanism has the effect of reducing the accuracy that the plow needs to be aligned when mating to the vehicle. The forward dimension is taken up by moving the vehicle forward into the plow. The remaining three dimensions will be forced into alignment to a small degree by the tapered entry into the latch.
The disadvantages of this mechanism are complexity and cost. This mechanism is also more complicated and therefore more costly than the standard coupling. It can be made rugged enough with the use of sufficiently strong parts. The plow frame must be held up from the floor manually to enable coupling.
Stanford addresses the problem of adjusting the height of the plow frame so as to align it with the vehicle frame. He proposes a screw jack that is attached to the plow frame and can be used to raise or lower the plow frame as needed. This device is used to align the plow frame in the vertical dimension.
The disadvantages of this invention are that it does nothing for alignment in the three other dimensions that must be done before coupling can be completed. There is some cost and additional complexity associated with the device.
Pester describes a device that acts as a carriage for the plow when it is not attached to the vehicle. This device should allow the accurate positioning of the plow frame to the vehicle frame with a minimum of muscle power. It encompasses a screw jack for raising the plow frame in the vertical dimension. There are castor wheels that would allow positioning the plow frame in the two rotational dimensions and the forward dimension.
The principle disadvantage of this device is that it requires a hard flat surface to roll upon. Snow plows are often left outside until needed. This means that they are covered with snow and ice and resting on frozen mud when needed. This device would be useless in these conditions. There is also the issue of the additional cost. Since the device is not attached to the plow it could get lost or stolen.
Hoekstra discloses a plow attachment method that uses the plow's lifting device to lower the plow frame onto a pin that is mounted on the vehicle frame. The plow frame ears have a slot that engages the pin on the vehicle frame. The joint secured by a second pin that is inserted above the coupling pin. This second pin prevents the plow frame lifting off the coupling pin during use.
This design produces a weaker joint that one with holes in the ears rather than slots. The slots can be forced opened by rough use. The plow is attached loosely by this method, the plow frame can move up and down the slots during use. This can result in damage to the ears.
The use of the lifting device aids in aligning the plow frame in the vertical direction and in the roll rotational dimension. The device does not aid in aligning the forward and back dimension or in the yaw rotational dimension. Therefore, the operator must manually move the plow into alignment before the slots will align with the pins. This can require considerable muscle power.